1. Field of the Invention
This invention relates to photopolymerizable compositions and particularly to negative-acting, photopolymerizable compositions useful in color prepress proofing constructions using a single receptor sheet to receive multiple color images on a single surface.
2. Background of the Art
Photopolymerizable compositions are used in numerous commercial applications. Negative-acting photopolymerizable compositions are used in presensitized printing plates (e.g., U.S. Pat. Nos. 3,218,167 and 3,887,450), prepress color proofing systems (e.g., U.S. Pat. Nos. 3,136,637; 3,671,236; and 4,482,625), and as resist films for etching and plating of substrates (e.g., U.S. Pat. No. 3,526,504).
Early photopolymerizable systems tended to have fairly slow photographic speeds and required long exposure under intense radiation to fully cure. This low speed was a function of low reactivity and few reactive groups in the monomers, poor radiation absorption and low efficiency in photoinitiators, and oxygen sensitivity in the acrylate functional polymerizable systems of choice. The speed of photopolymer compositions has been greatly increased by the use of monomers and coreactive oligomers with large numbers of reactive groups thereon (e.g., U.S. Pat. Nos. 3,827,956; 4,065,627; 4,153,778; 4,228,232; and 4,304,923), higher efficiency photoinitiators (e.g., U.S. Pat. Nos. 3,775,113; 3,729,313; 3,887,450; and 4,058,401), and the use of oxygen barrier topcoats such as poly(vinyl alcohol) and copolymers of acrylic acid (e.g., U.S. Pat. No. 3,895,949).
U.S. Pat. No. 4,228,232 shows a photopolymerizable composition which is particularly good, having both good photographic speed and oxygen insensitivity, without the use of polymeric barrier coatings on its surface. The composition comprises from 10 to 60% by weight of a polyfunctional photopolymerizable oligomer, 10 to 60% by weight of a photopolymerizable monomer, 10 to 60% by weight of a binder and 0.1 to 20.0 parts by weight of a photoinitiator system comprising a photoinitiator and sensitizing dye. This composition works quite well as a printing plate photosensitive composition. However, when used in a surprint color proofing system, there is very poor adhesive transfer of the image onto the receptor surface. The use of numerous types of binders is suggested.